ALEXANDRIA, Va. — Mayor Bloomberg’s charity is donating $50 million to the Sierra Club’s campaign against coal-fired power plants.

The billionaire mayor and Sierra Club Executive Director Michael Brune announced the partnership Thursday in Alexandria, Va.

Brune predicted that Bloomberg Philanthropies’ four-year commitment to the environmental organization’s Beyond Coal Campaign will help replace one-third of the nation’s aging coal-fired plants with clean energy, like wind or solar power, by 2020.

In a news release, Bloomberg said that while coal may seem to be an inexpensive energy source, the impact on the environment and public health is significant.

United Mine Workers President Cecil Roberts said it’s troubling that Bloomberg is willing to spend $50 million to help the Sierra Club put Appalachian coal miners out of work.